(1) Field of the Invention
The present invention relates to a developing device and an image formation apparatus provided with the same.
(2) Description of the Related Art
Image formation apparatuses, which include copiers and printers, are provided with a developing device that develops, by using developer, an electrostatic latent image formed on an image carrier such as a photosensitive drum.
A developing device usually includes a developing roller for carrying developer, and a circulation passage for circulating the developer with the aid of, for example, a transport screw provided with a spiral blade.
The developing roller faces the photosensitive drum, and develops an electrostatic latent image formed on the photosensitive drum, by using developer carried thereby and toner.
In many cases, the circulation passage for circulating developer has the following structure. The circulation passage consists of first and second transport passages and first and second communication passages. The second transport passage transports developer in the opposite direction as the first transport passage. The first communication passage connects a downstream section of the first transport passage with an upstream section of the second transport passage. The second communication passage connects a downstream section of the second transport passage with an upstream section of the first transport passage. Developer thus circulates via the circulation passage, and is supplied to the developing roller which is disposed along the first transport passage.
Each of the first and second transport passages is provided with a transport screw for transporting developer. However, neither of the first and second communication passages is provided with any transport screw. Developer circulates due to the flow force caused by the transport screws provided in the first and second transport passages.
With such a structure for circulation, developer often accumulates in the communication passages, which makes the surface of the developer uneven. That is, in each of the first and second transport passages, the surface level (i.e. height) of the developer is higher in its downstream section than in its upstream section.
Such an uneven surface level of the developer leads to an imbalance between the amounts of the developer at both edges of the developing roller in the direction of the axis thereof. This is problematic because, for example, there is a risk of degradation in the developing performance at one edge of the developing roller that is provided with a smaller amount of developer than the other edge.
Japanese Patent Application Publication No. 2007-334101 discloses a structure for making the surface level of developer even. According to Japanese Patent Application Publication No. 2007-334101, in both the first and second developer transport passages horizontally arranged next to each other, the pitch of the spiral blade of the transport screw is increased only near the downstream edge, so that the speed of transporting the developer is higher near the downstream edge than in the rest of the transport passage.
With this structure, the speed of transporting the developer is increased near the downstream edges of the transport screws. That is, the moving speed of the developer increases immediately before it enters the communication passages, and this flow force allows the developer to easily pass through the communication passages. This prevents the developer from accumulating in the communication passages.
In the meanwhile, there are great demands for downsizing image formation apparatuses, since image formation apparatuses are installed in offices and the likes. One possible means for downsizing an image formation apparatus is to downsize its developing device which serves as the main part of the image formation apparatus.
Usually, developing devices have an elongated shape along the main scanning direction (i.e. the direction that is orthogonal to the transport direction of printer sheets). Thus, to downsize a developing device, the length of the housing and/or the width of the housing can be shortened.
However, it is almost always necessary not to change the functionality related to image formation, even in the case of downsizing the image formation apparatus. The functionality means, for example, the system speed and the available sheet sizes. To meet such a condition, it is impossible to shorten the length of the housing. A possible way is to furthermore shorten the width of the housing even though it originally has an elongated shape.
Shortening the width of the housing means decreasing the capacity of the housing, and thus it is necessary to decrease the amount of the developer to be housed therein.
If the system speed is the same as before the downsizing, it is necessary to provide the photosensitive drum with the same amount of developer per unit time in order to keep the developing performance at the same level as before the downsizing.
One possible means for providing the photosensitive drum with the same amount of developer per unit time with a smaller total amount of developer in the housing is to increase the circulation speed of the developer in the housing and thereby provide the developing roller with an increased amount of developer per unit time. One possible means for increasing the circulation speed of the developer is to increase the rotation speed of the transport screw.
However, the flow of the developer becomes more turbulent as the rotation speed of the transport screw increases, and the transport amount per unit of the developer does not change linearly according to the change of the rotation speed of the transport screw. Thus, it is difficult to increase the circulation speed of the developer simply by increasing the rotation speed of the transport screw.
Furthermore, it should be noted that toner consumed in the developing operation is repeatedly refilled, and the amount of developer in the housing repeatedly increases and decreases due to the time lag between the consumption and the refilling.
Such changes of the amount of developer in the housing lead to changes of the amount of developer to be supplied to the developing roller. If the total amount of developer in the housing is relatively large, the change of the amount does not affect the amount of developer to be supplied to the developing roller very much, since the ratio of the change to the total amount is small.
However, in the case where the total amount of the developer is small due to the downsizing, if the amount of the developer changes by the same amount as before the downsizing, the ratio of the change to the total amount is relatively large. As a result, the developing roller tends to be short of developer when the amount of developer is decreased. The shortage of developer supplied to the developing roller leads to degradation of the developing performance such as lowering of density in high-density images.
Moreover, refilled toner should be stirred and charged while being transported. If the rotation speed of the transport screw is increased, the moving speed of developer is increased accordingly, and developer tends to be supplied to the developing roller before the charge amount reaches a required level. Shortage of the charge leads to degradation of the developing performance, such as causing fogging in blank space on printing sheets.
With the developing device disclosed in the Japanese Patent Application Publication mentioned above, it can be assumed that the surface level of the developer decreases overall in both the first and second transport passages when the total amount of developer is decreased. Here, if the amount of developer is changed under the condition where the total amount of developer is small, the ratio of the change to the total amount is relatively large. As a result, the developing roller tends to be short of developer. Also, when the transport speed of the developer is increased, the developing roller tends to be supplied with toner that is not charged enough. This is problematic.
Such a problem often arises particularly when a trickle developing method is adopted. In a trickle developing method, developer containing new carrier is refilled from an inlet of the housing bit by bit, and a portion of an excessive developer generated due to the refilling is discharged from an outlet of the housing as the developer overflows. Therefore, in developing devices in which a trickle developing method is adopted, the total amount of the developer in the housing is liable to change.